Epigenetic modifications play a crucial role in cellular differentiation and have been implicated in numerous disease states including cancer. One of the most studied of these modifications is the addition of a methyl group on the 5-position of the cytosine (5mC) base in a CpG dinucleotide. Accumulation of methylation marks in CpG rich regions around the transcriptional start site of genes has been shown to be associated with alterations in chromatin organization ultimately leading to changes in locus specific transcriptional activity. Paradoxically, DNA methylation marks can be heritably maintained across cell division but can also be reversibly/dynamically altered to establish new epigenetic programs. However, major uncertainties remain on how cells can erase existing methylation marks.
The recent discovery of a group of enzymes of the ten-eleven translocated (TET) family that can specifically modify these DNA methylation marks by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) has added another dimension of complexity to our understanding of DNA methylation. It has been well established for decades that certain bacteriophages contain 5-hydroxymethylcytosine rather than cytosine in their genome to protect themselves from host-controlled nucleases. The presence of 5hmC in mammalian cells has historically been very controversial, and its role in mammalian genomes is not well understood. Interestingly, Penn et al. demonstrated in 1972 that 5hmC can be detected by crude chromatography methods in rodent brain and liver DNA preparations. More recently, using mass spectrometry, Kriaucionis and Heintz provided firm evidence for the presence of 5hmC in Purkinje cells of the murine cerebellum. Subsequently, several studies have addressed the potential role of 5hmC and the oxidizing enzymes of the TET protein family in genome organization and differentiation of murine embryonic stem (ES) cells. The tissue specific cellular distribution of 5hmC in normal adult tissues and neoplasia, however, has thus far not been well documented.